656
FXUS65 KGJT 171747
AFDGJT

Area Forecast Discussion
National Weather Service Grand Junction CO
1147 AM MDT Tue Sep 17 2024

.KEY MESSAGES...

- Widespread showers and storms are expected today as a system
  moves through the region.

- Some storms may be capable of producing hail, damaging winds
  and heavy rainfall.

- A cold front sweeping through the area will keep temperatures
  cooler than normal for the next several days.

- Expect a break in the action Wednesday and Thursday before
  another system impacts the region Friday into the weekend.

&&

.SHORT TERM /THROUGH THURSDAY/...
Issued at 209 AM MDT Tue Sep 17 2024

Today a deep low pressure is going to track from Nevada to northern
Utah and eventually Wyoming. Ahead of that system is strong
southerly flow, which continues to advect moisture in from Arizona
and New Mexico. Scattered showers and storms have already developed
due to this advection and will continue across the area through the
morning. As the low pressure passes to our north it pushes a cold
front through the region. The system will provide impressive lift
for warm sector showers and the front will be a focus for additional
convective development later today. This development looks to occur
near the UT/CO state line later this morning. As the front sweeps
eastward so do the storms. Given the modest shear in place a few
storms could become strong to severe and capable of producing
damaging wind gusts. Even without showers the wind gusts along and
behind the front could reach 40-50 mph in spots as cold air aloft
allows for steep low level lapse rates and mixing down of the high
winds aloft. Models are hinting at pockets of CAPE that could
support larger hail, but that may depend on breaks in the clouds
ahead of the front. A few snow showers are possible on the higher
peaks as snow levels drive down to 11-12 kft. Most of the showers
end by sunset as the front clears the Divide. Subsidence and a dry
slot behind the front should limit any convection. Winds gradually
decrease this evening and cooler temperatures advect into the
area. Overall lows tonight end up at least 10 degrees cooler
than this morning. Tomorrow the weather is relatively quiet as
we will be in between two low pressure systems. High temperatures
are actually going to be below normal by 5-10 degrees.

&&

.LONG TERM /THURSDAY NIGHT THROUGH MONDAY/...
Issued at 209 AM MDT Tue Sep 17 2024

Models appeared to be in excellent agreement Thursday and Thursday
night but begin to diverge on Friday and by Sunday consensus was on
the low end of fair. The pattern on Wednesday features a closed mid-
level low over eastern Montana with another closed low spinning just
off the coast of San Francisco. Between these systems relatively dry
southwest flow moves over the forecast area with PWAT values near
0.3" for all but a small portion of the eastern Uinta Mountains
where values were near 0.5" according ensemble means. As a result,
skies will be partly cloudy/mostly cloudy over the Uintas Wednesday
with a slight chance for afternoon showers/thunderstorms. Warm air
advection and sunshine will help temperatures rise toward normal
late Wednesday afternoon. On Thursday models move the Pacific low
southeastward, though the ECMWF keeps it centered closer to the
coast just west of Bakersfield. The Canadian and GFS shift it
farther east along the border of Nevada northwest of Las Vegas.
Despite these differences, the forecast looks similar to Wednesday
with slight chances redeveloping for the eastern Uintas Thursday
afternoon while dry weather continues elsewhere. Temperatures climb
to near normal Wednesday and may rise a degree or two above the norm
on Thursday.

As noted previously, models fall further out of sync Friday, but
regardless, the region will see a modest increase in moisture with
PWATs climbing to near 0.5" for much of the area by midday. This
should be sufficient to generate low end scattered afternoon/evening
storms over higher terrain. The main threats will be gusty outflow
winds and lightning as the subcloud was indicated to be dry.
Nocturnal storms are likely Friday night as the low moves across
northern Arizona providing dynamic lift, though timing does vary
between models, so don`t want to get too hung up on positioning of
best chances. Looking at NBM compared to operational runs, it does
appear the blends are chasing the latest guidance a bit and won`t be
surprised to see the PoP field shift toward the southern zones in
later runs. By Saturday, positioning of the low is clearly up for
debate, though Canadian and GFS solutions were more similar. The
Canadian moves the closed low to the northeast Colorado Plains while
the GFS fills the low, though trough positioning was similar. In
contrast, the ECMWF carries the closed low over southwest Colorado
Monday afternoon. So again, the forecast is fairly generalized with
showers favoring the north in the NBM and while the ECMWF favored
the south and the Canadian and GFS were fairly equally distributed
along the Continental Divide. With the EC solution, showers may
linger overnight and persist into Sunday across the north as the low
center hurdles the Divide and moves to northeast Colorado then to
the northern High Plains on Sunday. The other models push the low
farther east over the central Plains and suggest precipitation
chances end ahead of midnight Saturday with potentially enough
lingering moisture for a few storms along the Continental Divide
Sunday afternoon.

Temperatures dip with increased clouds and showers and the trough
passage this weekend, running 3 to 5 degrees under seasonal norms
with the coolest temperatures arriving Saturday. Meanwhile,
overnight lows are expected to run within a few degrees of normal
throughout the long term period.

&&

.AVIATION /18Z TAFS THROUGH 18Z WEDNESDAY/...
Issued at 1132 AM MDT Tue Sep 17 2024

Scattered showers and thunderstorms continue to move west to
east through the region tracking to the northeast producing
brief periods of MVFR and outflow gusts to 50 kts. These storms
are generally along and east of the line KHDN-KDRO and will
slowly move east of the Divide over the next three hour to the
north and over the next six hours to the south. Otherwise,
expect VFR conditions with gusty southwest winds shifting
westerly with a frontal passage about three hours west of the
thunderstorms. Winds drop off overnight becoming light terrain
driven through the remainder of the TAF period.

&&

.GJT WATCHES/WARNINGS/ADVISORIES...

CO...None.
UT...None.

&&

$$

SHORT TERM...KJS
LONG TERM...NL
AVIATION...DB